The MOSFET as a voltage controlled resistor - Example

A MOSFET (L = 1 mm, t_ox = 10 nm, V_T = 1 V and m_n = 300 cm²/V-sec) is to be used as 50 W terminating resistor when applying a gate-source voltage, V_GS = 5 Volt. How wide should the gate be?
Answer:

The resistance of a MOSFET operated in the linear mode is given by:

R = V_DS/I_D

where the drain-source voltage, V_DS, and the drain current, I_D, are related by:




This equation can be solved for the gate width yielding:

W = L/( m C_ox R (V_GS - V_T )) = 10^-4/(300 x 3.47 x 10^-7 x 50 x (5-1)) = 48.3 micron.